Variable time discriminator for double frequency encoded information



United States Patent r 3,395,355 VARIABLE TIME DISCRIMINATOR FOR DOUBLE FREQUENCY ENCODED INFORMATION Andrew Gabor, Huntington, N.Y., assignor t0 Potter Instrument Company, Inc., Plaiuview, N.Y., a corporation of New York Filed Apr. 16, 1964, Ser. No. 360,314 7 Claims. (Cl. 328-72) ABSTRACT OF THE DISCLOSURE The specification and drawings disclose a decoding circuit for double frequency encoded information in which the period of the time discriminating circuit is varied in accordance with the frequency of the clock pulses.

This invention, generally, relates to a system for recovering information from signals read out from binary magnetic recordings and, more particularly, to an improved information recovery system of the type in which clock pulses and information are identified by means of a time interval discriminator.

In magnetic recording of binary signals, there has been considerable effort to increase the density of the recorded information so that more information can be recorded in smaller space on a magnetic tape or drum. One such high density recording system is disclosed in copending application Ser. No. 2 6,538, filed May 3, 1960, now US. Patent No. 3,217,329, and assigned to the same assignee as the present invention. This recording system is selfclocking, which means that the clock signals are interleaved with or are part of the recorded information pulses.

In high density recording systems which are self-clocking such as the system disclosed in the above-mentioned copending application, the successful recovery of information from the recorded signals depends upon the ability of the system to distinguish between long and short time intervals, because the clock pulses are distinguished from the information pulses by means of such time interval discrimination. The degree of difficulty of the time interval discrimination depends upon the nominal ratio of the long time interval to the short time interval, and the amount of variation depends upon the actual lengths of the long and short time intervals. The greater the ratio of the long time interval to the short time interval, the easier the discrimination will be.

However, the ability to discriminate depends ultimately upon what the minimum value of the ratio is, when the maximum variation of the long and short time intervals is considered. For example, if the nominal long interval is 10 microseconds and the nominal short interval is 5 microseconds, then the nominal ratio between the long and short time intervals is 2: 1, which is a value that would present no discrimination problems. However, if the long and short time intervals had varations of up to :2 microseconds, the short time interval would sometimes increase to 7 microseconds and the long time interval would sometimes decrease to 8 microseconds.

Thus, the minimum value of the ratio between the long and short time intervals would be 8:7, which is hardly detectable. If the varation were :25 microseconds, the discrimination would be impossible even in theory. Thus, it is apparent that the magnitude of variation of the long and short time intervals must be minimized.

There are two types of time interval variation which occur when a self-clocking magnetic recording is read out. In one type of variation, the full range of the variation may be encountered from one pulse interval to the next. This type of variation is either entirely random in nature or is related to the specific information content.

Another type of variation which occurs is one which changes gradually, but which may be extensive over a large number of intervals. There is relatively little change in this type of variation from one time interval to the next.

The system of .the present invention is designed to minimize the effect of the latter type of varation, which shall be referred to as long term variations.

In accordance with the present invention, therefore, a signal is produced which varies in accordance with the long term variation, and a time interval discriminator is controlled in accordance with this signal. The signal which varies in accordance with the long term time interval variation is produced, in the preferred embodiment, by generating a continuous train of constant width pulses, one for each clock pulse in the signal pulses read out from the magnetic tape or drum.

These constant width pulses are then integrated by a chain of integration stages to provide a signal inversely proportiional to an average of the time intervals between clock pulses, which average is weighted .in favor of the more recent intervals. This signal then is used to control the time interval discriminator.

In a preferred form of the invention, the integration stages each comprise a resistor and capacitor and the stages are interconnected by electronic buffer units to form a chain. The pulse train is fed into the input of the first integration stage, and the output signal, which will be inversely proportional to a weighted average of the time intervals between the clock pulses, will be taken from the last integration stage. The long term variation increases and decreases with the average time interval between clock pulses, so the output signal from the last integration stage of the chain of integration stages will vary in accordance with the long term variation.

Accordingly, an object of the present invention is to facilitate the information recovery from a high density self-clocking magnetic recording of binary signals.

Another object of this invention is to facilitate the discrimination between clock pulses and information pulses in signals read out clocking, magnetic recording of binary information.

A further object of this invention is to minimize the effect of long term variation in a time interval discriminating system for distinguishing between clock pulses and information pulses.

A still further object of the present invention is to provide an improved information recovery system.

Further objects and advantages of the present invention will become readily apparent as the following detailed description of the invention unfolds and when taken in conjunction with the drawings, wherein:

FIG. 1 illustrates waveforms of clock and information pulses read out from one form of a high density self-clocking magnetic recording of binary information;

FIG. 2 is a schematic circuit diagram of the system of the present invention; and

FIG. 3 is a graph showing several curves illustrating the operation of a system in accordance with the principles of the present invention.

The pulse train 11 in FIG. 1 is the form in which the pulses are produced by the read out circuit in the system disclosed in the above-mentioned copending application. In the pulse train 11, the clock pulses are designated generally by the reference number 13, and the information pulses are designated generally by the reference number 15.

The clock pulses 13 nominally occur at a constant frequency so that the spacing between the pulses 13 is shown to be constant. The information pulses 15 occur between the clock pulses 13, and the presence of an information from a high density, selfpulse between two clock pulses indicates the presence of a binary one in the information bit defined between the two adjacent clock pulses. The absence of an information pulse 15 between two adjacent clock pulses indicates the presence of a binary zero in the information bit defined between the two adjacent clock pulses.

The binary number represented by the information pulses 15 in the pulse train 11 is 0110100, as indicated directly below the pulse train 11. The information pulses are distinguished from the clock pulses in the signal train 11 by the time intervals between the pulses. Thus, if the next pulse occurring after a clock pulse 13 occurs after a short time interval, the pulse is an information pulse 15, but if the next pulse after a clock pulse 13 occurs after a long time interval, the pulse is another clock pulse 13.

Accordingly, the system requires a time interval discriminator to distinguish between clock pulses and information pulses.

In the system of the present invention shown in FIG- URE 2, a time interval discriminator is designated generally by the reference number 17. Signal pulses from a read out circuit, not shown but exemplified by the pulse train 11 in FIG. 1, are fed to the time interval discriminator 17 at an input 19. The time interval discriminator 17 then separates the clock pulses from the information pulses and produces the clock pulses on an output 21 and the information pulses on an output 23. The waveform 24a in FIG. 1 illustrates the clock pulses produced on the output 21, and the waveform 24b illustrates informa tion pulses produced on the output 23 as a result of the waveform 11 being fed into the input 19 of the discriminator 17.

The discriminator 17 performs the function of separating the clock and information pulses by comparing the length of the time interval between each clock pulse and the next succeeding pulse with a standard time interval. If the time interval is shorter than the standard, the succeeding pulse is identified as an information pulse and is transmitted to the output 23. On the other hand, if the time interval is longer than the standard, the succeeding pulse is identified as another clock pulse and is transmitted to the output 21.

In accordance with the present invention, the length of the standard time interval, with which the interval between each clock pulse and the next succeeding pulse is compared, is controlled by a signal voltage applied to the discriminator 17 at an input 25. The signal voltage applied at the input 25 is controlled to be inversely proportional to an average of the time intervals between the clock pulses, which average is weighted in favor of the intervals between the more recent clock pulses.

The signal voltage applied to the input 25 is generated by applying the clock pulses produced on the output 21 to a monostable multivibrator 27, which produces a train of square wave output pulses, one for each applied clock pulse. The output of the monostable multivibrator 27 is connected through a resistor 29 to one side of a capacitor 31, the other side of which is grounded. The RC time constant of the resistor 29 and the capacitor 31 is long compared to the width of the output pulses of the monostable multivibrator 27, so that the voltage produced across the capacitor 31 is an integration of the applied pulses.

The signal voltage produced on the capacitor 31 is transmitted by an electronic buffer 33 to a series circuit of a resistor 35 and a capacitor 37 connected between the output of the electronic buffer 33 and the ground and having the same values as the resistor 29 and the capacitor 31, respectively. With the resistor 35 and the capacitor 37 connected to the output of the electronic buffer 33 in this manner, the voltage on the capacitor 31 will charge the capacitor 37 through the resistor 35.

The voltage produced across the capacitor 37 is applied to the input of an electronic buffer 39 which transmits the voltage to its output. The output of the electronic buffer 39 is connected to ground through a series circuit of a resistor 41 and a capacitor 43, which have the same values as the resistor 29 and the capacitor 31, respectively. Thus, the voltage on the capacitor 37 will charge the capacitor 43 through the resistor 41.

Each of the capacitor and resistor combinations 29 and 31, and 37, and 41 and 43 is in effect an analogue integrating circuit, and the combinations are referred to as analogue integrating stages. These analogue integrating stages are connected in series into a chain by the electronic buffers 33 and 39. A plurality of additional identical analogue integrating stages are connected in the chain by additional electronic buffers following the stage comprising the resistor 41 and the capacitor 43. Only the last of these stages comprising a resistor 47 and a capacitor 49 is shown in FIG. 2 together with a buffer 50 which applies the voltage from the preceding integrating stage to the last stage.

The electronic buffers interconnecting the analogue integrating stages function to transmit the voltage from the capacitor of the preceding stage to the succeeding stage while preventing the succeeding stage from affecting the preceding stage. The buffers may be, for example, cathode followers. With the integrating stages connected in this manner, the signal voltage produced on the last capacitor 49 of the series of stages will be inversely proportional to an average of the time intervals between clock pulses with the average weighted in favor of the more recent time intervals. This signal voltage is applied by an electronic buffer 51 to the input 25 of the time interval discriminator 17 to set the standard time interval for the discriminator 17.

In the preferred embodiment, the number of integrating stages is five, but to obtain a better weighted average, the number of integrating stages can be increased, as desired. Instead of the above system, a system could be designed to provide a signal inversely proportional to the unweighted average of a predetermined number, for example ten, of the most recent time intervals between clock pulses. Such a signal would provide excellent results, but it would be a great deal more expensive than the system shown in FIG. 2, which by having sufiicient integrating stages connected in the series also provides excellent results. For this reason, the system shown in FIG. 2 is preferred over that in which a signal is produced inversely proportional to the unweighted average of the most recent time intervals.

The curves in FIG. 3 illustrate how well the preferred embodiment of the present invention using five integrating stages performs and shows how increasing the number of integrating stages improves the average obtained. The curves shown in FIG. 3 are signal voltages produced at the output of the chain of analogue integrating stages with each curve being the output voltage produced with a different number of integrating stages. The curves represent how the output signal voltage varies with time, starting with time zero when the capacitors of all the analogue integrating stages have a zero charge and the first clock pulse is applied to the input of the monostable multivibrator 27.

The curve 53 illustrates how the output signal voltage of the chain of analogue integrating stages varies with time when the number of analogue integrating stages is five. The curve 55 represents the output voltage that is produced with only four stages of the type identical to the five that produced the curve 53.

Similarly, the curve 57 shows the output signal voltage with three stages, the curve 59 illustrates the output voltage using only two stages, and the curve 61 shows the output voltage with only one stage. The curve 57 is actually the voltage that would be produced across the capacitor 43, the curve 59 represents the voltage that would actually be produced across the capacitor 37 and the curve 61 represents the voltage that would be produced across the capacitor 31.

The units of the time coordinates in FIG. 3 equal the RC time constant of one integrating stage, and the units of the voltage coordinates are selected so that the curves approach unity. It will be observed from the curve 53 that the output signal voltage will vary little after two units of time yet is virtually at its steady state value after ten units of time.

Thus, the instantaneous or random variations in the time intervals between clock pulses will have very little effect on the signal voltage produced across the capacitor 49 and applied to the input 25, whereas good averaging is obtained for the last ten units. From the remaining curves 55, 57, 59 and 61, it will be apparent how these desirable results are gradually lost as the number of analogue integrating sections is reduced.

The pulses from the read out circuit applied at the input 19 and exemplified by the pulse train 11 in FIG. 1 are applied to two gates 63 and 65 in the time interval discriminator 17. The system operates to enable the gate 63 and disable the gate 65 when the applied pulse is a clock pulse and passes the clock pulse to the output 21 and to the input monostable multivibrator 27. The system operates to enable the gate 65 and disable the gate 63 when the applied pulse is an information pulse so that the information pulse is passed through the gate 65 to the information output 23.

Each clock pulse passing through the gate 63 is applied to the input of a saw tooth generator 67 starts to generate a new saw tooth each time a new pulse is applied to its input. Therefore, the saw tooth wave form produced by the saw tooth generator 67 is synchronized with the clock pulses passing through the gate 63. The saw tooth waveform produced by the generator 67 is a negative going positive potential; that is, it starts out from a predetermined voltage and decreases linearly with time until the next clock pulse is applied, whereupon it steps immediately back to the predetermined starting voltage.

The saw tooth voltage waveform produced by the generator 67 is applied to one input of a voltage comparison circuit 69. The signal voltage applied at the input 25 is applied to another input of the voltage comparison circuit 69, and when the output signal voltage of the saw tooth generator 67 is higher than the signal voltage applied at the input 25, the comparison circuit 69 will enable the gate 65 and disable the gate 63. On the other hand, when the output signal voltage of the saw tooth generator 67 is below the signal voltage applied at the input 25, the comparison circuit 69 will enable the gate 63 and disable the gate 65.

The pulses produced by the monostable multivibrator 27 are selected to have a pulse width and amplitude so that the signal voltage produced at the input 25 will be between the maximum and minimum voltage produced by the saw tooth generator 67. The maximum voltage produced by the saw tooth generator 67 will, of course, be the predetermined voltage back to which the output voltage of the generator 67 is set each time a clock pulse is applied to the input of the generator 67. The minimum voltage of the saw tooth generator 67 will vary with the time between clock pulses.

The saw tooth generator 67 is designed so that its output voltage will normally pass the signal voltage applied at the input 25 after three quarters of the time interval between successive clock pulses has expired. That is, these conditions will occur when clock pulses are being read out at the designed or normal rate. Thus, under normal operating conditions, the gate 65 will be enabled and the gate 63 will be disabled for the first three quarters of each time interval between successive clock pulses, and any information pulse which occurs during the first three quarters of the time interval between successive clock pulses will pass through the gate 65 to the output 23.

In the last quarter of each time interval between successive clock pulses, the gate 63 will be enabled and the gate 65 will be disabled. Thus, the clock pulses applied at the input 1-9 will pass through the gate 63' to the output 21, to the input of the monostable multivibrator 27 and, at the same time, to the input of the saw tooth generator 67. Accordingly, the standard time interval, with which the time interval between each clock pulse and the next succeeding pulse is compared, is the time interval between the start of the saw tooth waveform and the time the saw tooth waveform passes the signal voltage applied at the input 25.

If the pulse succeeding a clock pulse occurs before the end of this standard time interval, the gate 65 will be enabled and the pulse will pass to the output 23 and, thus, be identified as an information pulse. If the pulse succeeding a clock pulse occurs after this standard time interval, the gate 63 will be enabled and the pulse will pass to the output 21 and, thus, be identified as a clock pulse.

Should the rate of the clock pulses increase, thus decreasing the time interval between successive clock pulses, the chain of analogue integrating stages will increase the signal voltage applied at input 25 so that the output voltage of the saw tooth generator 67 will drop down to the signal voltage applied at input 25 after a shorter time interval. Therefore, the standard time interval for the discriminator 17 is reduced. Similarly, if the clock pulse rate should decrease from its normal rate, the time period between successive clock pulses will increase.

The signal voltage applied at the input 25 by the chain of analogue intergrating stages will decrease so that the length of time that it takes the saw tooth waveform to decrease to the value of the signal voltage applied at input 25 will be increased and, thus, the standard time interval for the discriminator 17 will be increased. When the average length of time intervals between successive clock pulses increases, the standard time interval for discriminator 17 is increased, and when the average length of the time intervals between successive clock pulses de creases, the standard time interval for the discriminator 17 is decreased. The amplitude and the length of the pulses produced by the multivibrator 27 is selected so that the voltage applied at the input 25 will vary the standard interval for the discriminator 17 to always be approximately three quarters of the average length of the interval between the :most recent clock pulses.

In this manner, the effect of long term variation in the long and short time intervals is greatly minimized. The above description is of a preferred embodiment of the invention, and many modifications may be made thereto without departing from the spirit and scope of the invention, which is defined in the appended claims.

What is claimed is:

1. An information recovery system for recovering information signals from a train of signals in which information is encoded as the presence or absence of a signal between regularly occurring clock signals comprising,

a time interval discriminator connected to receive the train of signals,

said time interval discriminator producing a first output signal in response to each clock signal a first interval after each clock signal,

means responsive to said first output signal for coupling information signals from said train of signals to an output terminal, means responsive to said clock signals for producing a second output signal that is a function of the average frequency of said clock signal over a second interval that includes a plurality of clock pulses,

means for coupling said second signal to said time interval discriminator, and

said time interval discriminator responsive to said second signal to vary said first interval so that said first interval increases when said clock signal frequency decreases and said first interval decreases when said frequency increases.

2. An information recovery system for recovering information signals from a train of signals including clock signals comprising,

a time interval discriminator connected to receive said train of said signals including means to compare time intervals occurring in said train of signals with a standard time interval, a control input to said time interval discriminator including means to vary said standard time interval in accordance with the signal voltage applied to said control input,

a plurality of analogue integrating stages connected into a chain,

means to apply a constant impulse content pulse to the first stage of said chain of analogue integrating stages in response to each clock signal in said train of signals, and

means to apply the voltage produced in the last stage of said chain of analogue integrating stages to the control input of said time interval discriminator.

3. An information recovery system for recovering information pulses from a train of pulses including clock pulses comprising,

time interval discriminating means operable to compare the interval between each clock pulse in said train of pulses and the next successive pulse in said train of pulses with a standard time interval, said time interval discriminating means having a control input and being operable to vary said standard time interval in accordance with the signal voltage applied to said control input,

a plurality of analogue integrating into a chain,

means to apply a constant impulse content pulse to the first stage of said chain of analogue integrating stages in response to each clock pulse in said train of pulses, and

means to apply the signal voltage produced in the last stage of said chain of analogue integrating stages to the control input of said time interval discriminating means.

4. An information recovery system for recovering information signals from a train of signals including clock signals comprising,

a time interval discriminator connected to receive said train of signals and operable to compare time intervals occurring in said train of signals with a standard time interval, said time interval discriminator having a control input and being operable to vary said standard time interval in accordance with the signal voltage applied to said control input,

a plurality of stages each comprising a resistor and a capacitor connected in series,

a plurality of electronic buffers connecting said stages into a chain so that the voltage across the capacitor stages connected in each preceding stage in said chain charges thecapacit-or in the succeeding stage through the resistor of the succeeding stage,

means to apply a constant impulse content pulse to the first stage of said chain in response to each clock signal in said train of signals so as to charge the capacitor of said first stage through the resistor of said first stage, and

means to apply the voltage produced across the capacitor of the last stage of said chain to the control input of said time interval discriminator.

5. An information recovery system as recited in claim 4 wherein said chain of stages has at least five stages.

6. An information recovery system for recovering information pulses from a train of pulses including clock pulses comprising,

time interval discriminating means to compare the time interval between each clock pulse in said train of pulses and the next successive pulse in said train with a standard time interval, said time interval discriminating means having a control input and being operable to vary said standard time interval in accordance with the signal voltage applied to said control input,

a plurality of stages each comprising a resistor and a capacitor connected in series,

a plurality of electronic buffers connecting said stages into a chain so that the voltage across the capacitor in each preceding stage charges the capacitor in the succeeding stage through the resistor of the succeeding stage,

means to apply a pulse of constant impulse content to the first stage of said chain in response to each clock pulse in said train of clock pulses so as to charge the capacitor of said first stage through the resistor of said first stage, and

means to apply the voltage produced across the capacitor of the last stage of said chain to the control input of said time interval discriminating means.

7. An information recovery system as recited in claim 6 wherein said chain of stages has at least five stages.

References Cited UNITED STATES PATENTS 3,321,706 5/1967 Donovan 328-127 3,304,437 2/1967 Dano 328-127 3,243,580 3/1966 Welsh 340l74.1 3,237,176 2/1966 Jenkins 340174.l 3,191,013 6/1965 Reader 328l09 BERNARD KONICK, Primary Examiner. A. I. NEUSTADT, Assistant Examiner. 

